This invention relates to the design of protective jackets for optical fiber transmission media.
Research and development work in the field of optical fiber transmission is advancing rapidly toward the first commercial use of optical fibers in communication systems. A variety of fiber waveguiding structures have been reported which are capable of transmitting large quantities of information via modulated optical wave energy in a single mode or multimodes with transmission losses as low as 2 decibels per kilometer. It is expected that such fibers will eventually be used much as wire pairs, coaxial cable and metal waveguides are now used in conventional lower frequency systems. Although a variety of fiber arrangements are presently being studied, it is likely that the earliest implementation will involve the use of a fiber cable which includes a plurality of optical fibers. Forming the individual fibers in closely packed, well-defined relative positions in a single cable enclosure will permit multiple channel transmission and, at the same time, will facilitate handling (e.g., splicing) of the fibers in the field.
For an excellent review of the state of the art of optical fiber transmission, reference is made to the article in Volume 61 of the Proceedings of the IEEE, pages 1703-1751 (December 1973).
Because of their relatively small size (i.e., overall fiber diameters typically range anywhere from about a few micrometers to 150 micrometers), optical fibers are inherently fragile structures. In most presently known fibers, surprisingly small external forces cause substantial distortions of the fiber's central axis. As shown in the paper by D. Marcuse in Volume 51 of the Bell System Technical Journal, pages 1199-1232 (1972), such distortions can cause coupling of the optical wave energy propagating in a fiber from one mode to another mode, with consequent optical energy loss. For example, it can be shown that minute irregularities in the machined surface of a metal drum on which a fiber is wound suffice to cause substantial distortion loss therein, even if the fiber is wound on the drum with only a few grams of tension. The forces exerted on the individual fiber in a cable enclosure will almost certainly be considerably stronger and less uniform than these drum forces. The concern with this problem has increased recently with the realization that the lowest loss values for any particular fiber are measured almost invariably in connection with an extremely small amount of mode coupling, and after carefully eliminating external forces from the fiber.
The broad object of the present invention is to provide a variety of arrangements for effectively shielding an optical fiber from the influence of external forces, and thus for minimizing distortion loss.